The present invention relates to a sintered oil retaining bearing suitable for use as a bearing that is built in small-sized spindle motors for information equipment, which is required to have high running accuracy, including, for example, optical disk devices, such as CD-ROMs and DVD-ROMs/RAMs, magnetic disk devices, such as HDDs, and polygon scanners for. laser beam printers, and it also relates to a method of producing the same and a spindle motor for information equipment.
Small-sized spindle motors for information equipment, which is required to have high running accuracy, including, for example, optical disk devices, such as CD-ROMs and DVD-ROMs/RAMs, magnetic disk devices, such as HDDs, and polygon scanners for laser beam printers, are required to have high speed and high running accuracy as their performance is enhanced.
For example, in the aspect of high speed, in the case of CD-ROMs, their rotary speed was thousands of rpm when first put to practical use but recently there has been a demand for a higher rotary speed of above 10,000 rpm. Further, in the case of polygon scanner motors, the required rpm was about 10,000 except for equipment that used hydrodynamic air bearings, whereas recently there has been an increasing demand for a higher rotary speed of about 30,000 rpm.
In such information equipment, the rotary shaft on which a rotary element, such as a disk, is mounted is supported usually by ball bearings and recently a support construction using sintered oil retaining bearings has been employed.
In this connection, in the aforesaid small-sized spindle motors, high running accuracy and reduced noise are simultaneously required in addition to high speed; therefore, it has become difficult for the heretofore employed ball bearings and sintered oil retaining bearings to satisfy the aforesaid demanded performance.
For example, with ball bearings used, the following drawbacks occur.
Ball bearings are accompanied by a peculiar race sound (the sound produced by balls rolling on the rolling bearing rings) and noise caused by the self-vibration of the cage, and if they are operated at high speed, the noise level rises, so that their application to polygon scanner motors and the like is not suitable.
Further, the ball bearing has many components, including an outer ring, an inner ring, balls, a cage, seals and grease; thus, many factors that affect running accuracy, including the mechanical accuracy of the respective components, the accuracy with which the ball bearing is assembled to a motor, and the pre-pressurized state, are complicatedly mixed up. Therefore, the management for accuracy is difficult and it has been hard to attain the running accuracy including NRRO required of magnetic disk motors.
On the other hand, the sintered oil retaining bearing is superior to the aforesaid ball bearing in that its noise and cost are low, but since it is porous, it has surface openings in the bearing surface, allowing the oil in the bearing clearance to escape into the bearing body when an oil film pressure develops, because of which construction if there is a variation in the size of the surface openings, the amount of the escaping oil differs from place to place, thus adversely affecting the rotary performance. Therefore, it has been difficult to apply conventional sintered oil retaining bearings to the equipment of which high running accuracy and the like are required.
An object of the invention is to provide a sintered oil retaining bearing that secures the stability of an oil film in the bearing clearance and achieves high running accuracy through management to exclude a variation in the size of surface openings in the bearing surface, particularly the presence of large holes, while making use of such advantages as the mass productivity, low noise and low cost features of sintered oil retaining bearings.
A sintered oil retaining bearing according to the invention has a bearing body that is composed of a porous body formed of a sintered metal and that is formed with a bearing surface opposed, through a bearing clearance, to the outer peripheral surface of a shaft to be supported, said bearing body being impregnated with lubricating oil or lubricating grease, said sintered oil retaining bearing being characterized in that the surface openings in said bearing surface are substantially uniform in size, and in that when the area of a single such surface opening is converted into the area of a circle, the diameter of such circle does not exceed 0.05 mm.
The area of a single surface opening in the bearing surface of a general sintered oil retaining bearing corresponds mostly to a diameter of 0.01-0.02 mm when it is converted into the area of a circle, but there are large holes whose area corresponds to a circle diameter exceeding 0.05 mm or 1 mm. If there are large holes whose area corresponds to a circle diameter exceeding 0.05 mm, the amount of the oil escaping into the bearing body is locally increased in the region where said large holes exist, causing trouble to the continuity of oil films and the stability of oil film pressure.
Therefore, if it is ensured that the respective areas of said surface openings are substantially uniform and that when the area of such surface opening is converted into the area of a circle, the diameter of the circle does not exceed 0.05 mm, it is possible to prevent the local escaping of oil, improving the continuity of an oil film in the bearing clearance and the stability of oil film pressure, thus providing the effect of reducing the shaft deflection of, e.g., optical disk motors.
Further, if the proportion of the surface area occupied by the surface openings to the total bearing surface is 10% or less, the oil film pressure in the entire bearing clearance is improved and the bearing rigidity is increased, so that the bearing can be applied even if imbalance loading occurs due to the mounting of a disk as in optical disk devices or even if a high centrifugal force acts due to high speed driving.
In a sintered oil retaining bearing according to the invention, if the bearing body is formed with a plurality of bearing surfaces located in axially spaced places, and if the inner diameter of the region between bearing surfaces is set greater than the inner diameter of the bearing surfaces, the axial misalignment between bearing surfaces can be minimized when a plurality of bearings are separately incorporated into a spindle motor.
That is, a plurality of bearing surfaces are provided in a single bearing and since they are simultaneously formed, the axial misalignment between bearing surfaces can be minimized. Further, since the inner diameter of the region between bearing surfaces is set greater than the inner diameter of the bearing surfaces, there is no possibility of the torque increasing as compared with the case where a plurality of bearings are separately incorporated.
Further, if axially inclined hydrodynamic grooves are formed in the bearing surfaces of the bearing body, then such hydrodynamic grooves form satisfactory oil films in bearing clearances, and the resulting oil film pressure supports an object shaft in a contactless manner, greatly improving the running accuracy of the shaft.
In addition, a sintered oil retaining bearing according to the invention is incorporated into a spindle motor for information equipment that comprises a rotary shaft on which the rotary element of information equipment is mounted, a rotor mounted on said rotary shaft or a rotary member rotatable with said rotary shaft, and a stator mounted on a stationary member, and said bearing can be used as a bearing for rotatably supporting said rotary shaft.
Further, as a production method that ensures that the surface openings in the bearing surfaces are substantially uniform in area and that excludes large openings whose area corresponds to a circle diameter exceeding 0.05 mm when it is converted to the area of such circle, the present invention is characterized in that in compacting a metal powder into a cylindrical form by using a forming die and a forming core rod, a relative motion is imparted between the forming core rod and the metal powder after the latter has been charged into the forming die. Thereby, the bridge phenomenon that occurs when a metal powder is charged into a forming die can be excluded and the manufacturing process does not lead to a cost increase.